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Melting of residual eclogites with variable proportions of quartz/coesite

Anja Rosenthal1, Gregory M. Yaxley1, David H. Green1, Jörg Hermann1 and Carl S. Spandler2

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 School of Earth and Environmental Sciences, James Cook University, Townsville, Australia

Eclogite and pyroxenite layers and veins within the peridotitic mantle may be important in genesis of many magmas. Understanding high pressure melting of residual eclogites with varying amounts of quartz/coesite (qz/co) will improve understanding of the melting behaviour of heterogeneous mantle assemblages. We are investigating the phase and melting behaviour of a residual eclogite composition, which crystallizes garnet (grt) + clinopyroxene (cpx) at 3.0 & 4.0 GPa with no qz/co (REC). The melting behaviour of REC reveals the anhydrous solidus of a coesite-free eclogite.

The subsolidus phases of REC are garnet (grt) and clinopyroxene (cpx) at 3.0 & 4.0 GPa. The solidus is at 1325±25 °C at 3.0 GPa, and at 1475°C±25°C at 4.0 GPa. Melt compositions and proportions with cpx > grt are controlled by grt-cpx cotectics. At 3.0 GPa, cpx/grt ratios decrease with increasing % melting. At 4.0 GPa, cpx/grt ratios are lower at a given % melting than at 3.0 GPa.We are also studying eutectic-like melting in coesite-bearing eclogites with varying proportions of qz/co. REC10 and REC20 are identical to REC, but have 10% and 20% higher SiO2 contents, respectively.

Both REC10 & 20 crystallise sub-solidus as grt + cpx + qz/co at 3.0 GPa. The solidi of both are similar at 3.0 GPa, at 1275±25°C. Low-% melting (<20%) is eutectic-like until qz/co melts out. Like REC, grt-cpx cotectics control high degree melting for all compositions, with cpx abundance always exceeding grt. However, for REC, the proportion of cpx always exceeds those in REC10 & 20, but the % melt is always lower than for REC10 & 20 at given temperature. Melts vary with increasing % melting from andesitic to basaltic for REC & REC10, but from dacitic to basaltic-andesitic for REC20. At <25% melting, melt in REC has lower Mg# at given % melting than melt in REC10 & 20. The contrary is observed at higher % melting.

Garnet and cpx in REC have higher Mg# at a given % melting than grt and cpx in REC10 & 20. Towards higher % melting (>25% melting), variations in cpx Mg# diminish.
These variations of residual grt and cpx Mg# and melt Mg# are principally because melts from REC (qz/co-free) are more mafic at a given % melting. These outcomes reveal the strong dependence of Mg# of residual mineral phases & melt on the presence or absence of qz/co in melting of eclogites.

Rosenthal A, Yaxley GM, Green DH, Hermann J, Spandler CS (2008) Melting of residual eclogites with variable proportions of quartz/coesite. Geochimica et Cosmochimica Acta 72 (12): A806